In the filtration, particularly of expensive chemicals in semiconductor industry or the like, it is often necessary to filter a small amount of liquid, for example, about 1 liter and after the completion of filtration it is necessary to recover the residual liquid as much as possible. Particularly, in filtration operations which require a high precision, the size of the particles to be trapped is becoming smaller and smaller and an improved technique for processing a liquid containing such particles is correspondingly demanded. Performance of trapping object particles down to a particle diameter of 0.02 μm is presently required.
In the conventional filtering system which has a principal object of minimizing residual amount of liquid, a filter device including a disc filter (a disc-like filter membrane supported on a perforate support disc) is encased in a housing. However, with such conventional filter device, the filtering surface area of the disc filter is small relative to the size of the housing and according the filtering operation tends to require a long filtering time, because the filtering surface area is restricted to that proportional to square of the outer diameter of the disc filter.
As a measure, a filter device including two or more disc filters stacked on each other (stacked filter) encased in a housing can provide a filtering surface area which is a surface area of a single filter multiplied by the total number of the stacked disc filters, whereby the filtering time is reduced. In this case, however, the volume required for encasing the disc filters becomes larger with the increase of the number of the disc filters because the filter device requires flow passages between the inner surface of the housing for the stacked disc filters and the outer diameter of the stacked disc filters and between the discs, and thus the added volumes of the primary flow passage and the secondary passage become larger per one disc filter with the increase of the number of disc filters. Thus, there is a problem that the residual liquid amount of liquid remaining within these spaces in the housing is large at the time when the filtration of a small quantity of liquid has been terminated.
On the other hand, a filter device composed of a pleated filter, which is formed by pleating a filter membrane, forming it into a cylindrical form and inserting it in a housing, has an advantage that the filtration surface area can be made much larger than the disc filters for the same size of housing, and thus the time period for filtration operation becomes shorter. However, there is a structural disadvantage with respect to the residual quantity of the liquid under treatment.
For example, where a desired amount of liquid is treated within a predetermined period of time, the pleated filter can increase the filtering surface area without significantly increasing the volume of the flow passages and minimize the pressure loss. However, even with this type of structure, a perforate inner cylinder (core) and a perforate outer cylinder (sleeve) having a number of openings are usually required to be installed within a housing for supporting the inner and outer surfaces of the pleated filter and the core has an internal passage, and a space is formed between the sleeve and the cylindrical housing (container) to form a liquid passage for example, Japanese Publication No. 05-111622. These passages are disadvantageous because they form spaces for the stagnant residual liquid unfiltered as well as filtrate at the terminal point of filtration operation.
Especially for filtration of a small amount of liquid, a trial to enhance the filtration precision and to minimize the residual liquid within the filter housing requires a long period of treatment, while a trial to enhance the filtration precision and to minimize the time necessary for the treatment increases the residual liquid within the filter housing.
There is a need to filter small amounts of a liquid to enhance the filtration precision and to minimize the residual liquid within the filter housing. There is a need to filter small amounts of liquid to enhance the filtration precision, minimize the residual liquid within the filter housing, and minimize the filtering time.